The general aim of the research proposed herein is to gain a more complete understanding of the early embryonic origins of behavior in vertebrates and to attempt to relate functional changes during ontogeny to anatomical, physiological and neurochemical mechanisms in the central and peripheral nervous system. More specifically there are three experimental programs outlined here which, each in its own way, attempts to deal with this general problem. In the first place, there are experiments with the chick in which embryos at different stages of development will be prohibited from moving by injection of a neuromuscular blocking agent (curare). The embryos will be immobilized in this way for 1-3 days, allowed to recover from the drug, and their subsequent embryonic and posthatching behavior evaluated. Such a design should allow one to answer questions concerning the role of sensory experience and/or motor practice or activity in the ontogenetic organization of behavior. Still another approach to the general question posed above will be to use pharmacological and neurochemical techniques for determining: (a) the role of active inhibitory mechanisms in embryonic behavior, and (b) the role of biogenic amines in the embryonic posthatching behavior of the chick. And, as a final approach to this question, I would like to propose the initiation of a program to examine the development of behavior and the nervous system in the pouch-young opossum.